How can I use a Hidden Markov Model to recognize images?
You wouldn't, normally. A HMM is used to model sequences of observations, and it would not make sense to use it for image recognition. Unless they are sequential, such as strokes in handwriting.

HMMs are typically used in fields such as speech recognition and part-of-speech tagging. Here you observe a sequence of events that you want to fit to a model in order to classify the individual observations.

For training a HMM you would use something like the Baum-Welch Algorithm; for finding the most likely sequence (ie the recognition process) the Viterbi Algorithm is used.